1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece to be polished, and more particularly to a polishing apparatus having a film thickness measuring device for measuring a film thickness of a surface of a workpiece such as a semiconductor wafer.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. In the case of photolithography, which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because the depth of focus of an optical system is relatively small. A polishing apparatus for performing chemical mechanical polishing (CMP) has been used for planarizing such a semiconductor wafer.
In a chemical mechanical polishing process, after a surface of a semiconductor substrate has been polished for a certain period of time, the polishing process should be stopped at a desired timing or at the time when the semiconductor substrate has a desired film thickness. For example, some integrated circuit designs require an insulating film (layer) of SiO2 or the like to be left on a metallic interconnection of copper, aluminum, or the like. Since a metallic layer or other layers are further deposited on the insulating layer in a subsequent process, such an insulating layer is referred to as an interlayer. In this case, if the semiconductor substrate is excessively polished, then the lower metallic layer is exposed on the polished surface. Therefore, the polishing process needs to be stopped in a state in which a predetermined thickness of the interlayer remains unpolished.
According to another polishing process, interconnection grooves having a certain pattern are formed in a surface of a semiconductor substrate, then a Cu layer is deposited on the surface of the semiconductor substrate to fill the interconnection grooves with copper or copper alloy, and unnecessary portions of the Cu layer are removed by a chemical mechanical polishing (CMP) process. Specifically, the Cu layer on the semiconductor substrate is selectively removed by the chemical mechanical polishing process, leaving the Cu layer only within the interconnection grooves. More Specifically, the Cu layer is required to be removed so that an insulating layer of SiO2 or the like is exposed on surfaces other than the interconnection grooves.
In such a case, if the semiconductor substrate is polished so excessively that the Cu layer in the interconnection grooves is removed together with the insulating layer, then the resistance of the circuits on the semiconductor substrate would be so increased that the semiconductor substrate might possibly need to be discarded, resulting in a large loss of resources. Conversely, if the semiconductor substrate is polished so insufficiently as to leave the copper layer on the insulating layer, then interconnections on the semiconductor substrate would not be separated from each other as desired, and short circuits would be caused between those interconnections. As a result, the semiconductor substrate would be required to be polished again, and hence its manufacturing cost would be increased. The above problems also occur when another metallic film of aluminum or the like is formed on a semiconductor substrate and polished by a CMP process.
Therefore, it has been proposed to detect an end point of a CMP process with the use of an optical sensor. Specifically, an optical sensor having a light-emitting element and a light-detecting element is provided in a polishing apparatus. The light-emitting element of the optical sensor applies light to a polished surface of a semiconductor substrate, and the light-detecting element detects a change in reflectance of light reflected from the polished surface to measure the film thickness of an insulating layer or a metallic layer on the polished surface. Thus, the end point of the CMP process is detected from the measured film thickness.
However, a conventional film measuring device using an optical sensor measures the film thickness only at points on a line along a radial direction of the semiconductor substrate or at variable points. Specifically, the conventional film measuring device cannot measure the film thickness at fixed points. Therefore, it is difficult to highly accurately obtain the film thicknesses of the semiconductor substrate through continuous measurement. Further, a conventional polishing apparatus requires a separate film thickness measuring device for inspecting the quality of the polished semiconductor substrate, in addition to a film thickness measuring device for detecting an end point of the CMP process. Therefore, the conventional polishing apparatus needs a larger space for the additional film thickness measuring device, and thus the size of the apparatus becomes larger.